This application relates to measurements of surface slopes and other topological properties of surfaces in flat panels, substrates, and wafers, and more particularly, to optical techniques and systems for such measurements.
Optical interferometry uses optical interference between two at least partially mutually coherent beams to extract information embedded in the wavefront of at least one of the beams as an optical probe beam which interacts with a target whose information is under measurement. Various coherent optical measurement techniques have been proposed for measuring deformation fields on loaded and deforming solids with increased sensitivity, owing to the coherence property of lasers [1, 2], while several interferometry techniques, such as Moiré interferometry and speckle pattern interferometry, are widely employed in experimental stress/strain analysis [3, 4]. The suitability of these and other techniques for optical measurements depends on the optical properties of the object under measurement and the nature of the mechanics problems under investigation. The application of such techniques in deformation analysis often requires numerical differentiation of discretely-sampled displacement data which may introduce significant error magnification problems. In addition, many of these methods can be undesirably sensitive to rigid-body rotations and susceptible to ambient vibrations.
One of optical interferometry techniques for optical measurements is wave front shearing interferometry [5] for performing optical differentiations of wave-front phase by using self-referencing common-path interference between two laterally sheared wave-fronts. A typical optical shearing interferometer produces and interferes two spatially shifted replicas of the same, usually distorted wavefront of an optical beam along a direction transverse to the direction of propagation of the wavefront. The interference between the spatially shifted and replicated wavefronts generates an interference pattern representing the spatial distribution of slopes in the wavefront. In an effect, the shearing interferometry performs an optical differentiation of the wavefront and thus can be used to reduce the numerical differentiation of discretely-sampled displacement data and thus reduce errors associated with such numerical differentiation. Another feature of optical shearing interferomety is measurement of a deformation of one point of the wavefront to another of the same wavefront separated by the shearing distance, i.e., the distance between the two interfering replicas of the same wavefront. In this sense, an optical shearing interferometer is a self referencing interferometer and thus provides insensitivity or immunity to vibrations and other perturbations present at the wafer or device under measurement.
In implementations, a shearing interferometer may be configured to produce a shearing interference pattern from either of the optical transmission of the probe beam through the surface or from the optical reflection of the probe beam by the surface. The shearing interference pattern is then processed to obtain surface, slopes, curvatures and other surface topographical information. Examples of measurable surfaces include but are not limited to surfaces in various panels and plates, various substrates and wafers, integrated electronic circuits, integrated optical devices, opto-electronic circuits, and micro-electro-mechanical systems (MEMs), flat panel display systems (e.g., LCD and plasma displays), photolithography masks, pellicles and reticles. Optical shearing interferometry can be implemented in various configurations, including a coherent gradient sensing (CGS) system using optical gratings to cause the shearing of the wavefront (see, e.g., U.S. Pat. No. 6,031,611), a radial shear interferometers, wedge plate in a ai-lateral shearing interferometer (see, e.g., U.S. Pat. No. 5,710,631) and others.